My home's AC isn't cooling. The air coming out of the vents is the same as the return. The outside unit's fan is running. One of the lines seems to be icing over. I turned it all off to let it thaw and it was thawed on the outside at least.

What else can I look at before calling for emergency help?

Airflow

Cut it off and let the fan run without unit on.

Install new filter

Thanks. Should have mentioned that. The filter needed to be replaced and now has been. How long should I let it sit before I turn the compressor back on?

24 hours if the A/C coil is iced up. Low air flow or low refrigerant or both tends to be the reason Icing occurs. [hockey players will disagree]

Ouch. I was hoping it was more like thirty minutes. Thank you

That was my ''hook gauges up to it and check pressures time.'' Is there good air flow inside the house? If the A coil is all iced up and there isn't much air blowing then time is it's best friend.

Dealt with this over the weekend.  My own fault.

- replace air filter on your air handler.  From what I read, reduced air flow plays a large part.  Make sure your air filter is clean and make sure at least 75% of your vents are open.  My air filter was dirty and some vents were still shut off from wood furnace use last winter.

- Shut off power to the air handler (mine has a switch, you can also do it at the breaker box).  Your coil in the air handler is likely iced up as well.  Mine was.  Badly.  I pulled the access panel off the air handler, set a ladder up next to the coil, placed a small space heater on the ladder (18" - 24" from the coil) and let it melt all the ice off the coil.  You just want to get some warm air over the coils, not nuke them.  You've got a bit of time, because while this ice is melting you want to...

- Shut off the power to the compressor unit outside.  Usually there's a disconnect in the box it's connected to, or I guess you can do it from the breaker as well.  Then clean out the coils on the compressor unit outside.  I took apart the sides to have good access to the coils.  I then sprayed it from the inside out with a water hose.  Be careful not to bend the fins.  May need to rinse off the outside of the fins as well to clean off any debris.  Rinse until water is clear and free flowing through the fins.  Giving this a good cleaning really helps the AC with efficiency.  

You'll likely need to remove the fan on top.  Usually the leads on the fan are long.  You can trace them down to the contactor / solenoid (black and red) and capacitor (brown in my case).  Be careful removing and re-installing the wires!  The capacitor will hold a charge even when disconnected and can really nail you.  I just disconnected the fan and moved it off the the side on the lawn.

Your compressor and lines to it are probably frozen up as well.  I got all the ice off the compressor with the hose while I was washing the outside unit.

Once the compressor unit was all cleaned out I put everything back together.  I went inside and checked the air handler and the heater had melted all the ice off the A coil.  I put the access panel back on the air handler and had the wife fire up the AC on the thermostat.  It fired up and worked fine.

I don't know how long this process took.  It felt like half a day.  It was probably an actual hour or two.  
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I dealt with this last month as well

The blower fan in the furnace needed a new capacitor to spin full speed.

Super cheap and easy fix.

Turn “fan” on
No ac
This will run air through the handler, accelerating the thawing process.
In this heat, I’d let it run 30min

In the meantime, be sure your drain pan pipe and overflow are clear.

You should get really good vent airflow by then.

Turn it back on.

No airflow at all. Time it is.

There’s no harm in using a hairdryer to speed things along.

If it ices up with a clean coil and clean filter and air is coming out the vents,
Then call someone with the tools and knowledge.

I've also seen it be a dirty, packed-up evaporator coil.

Simply putting a small fan on it will speed up the process.

That will make a lot of fluid, make sure all flow to the condensate drain is clear.  Mine clogged last weekend.  Still blew cold, but the main manifold/register for duct output started sweating and damaged my ceiling. You are already in there, make sure the coils are clean.

I have defrosted coils with a good sized heat gun.  

Serious question - if the air handler side is iced up, would there be any harm in turning the heat pump to heat mode for a few minutes to melt the ice, and then running the fan for 15 minutes or so to dry it out before turning it back to cooling mode?

Go ahead, no issue.

issue with killing power is if the unit has a condensate pump.   With ice you’re going to get an overflow if you have a condensate pump and no power.  Safer if you don’t know to just put it on fan and defrost the whole system that way n

I had an icing problem a few years ago. I was using a new expensive filter that could filter pollen etc.

Even though it was almost brand new, it didn’t flow as much air as the cheap fiberglass filter.

Went back to the cheap filter and my icing problem went away.

Good point.  Mine is gravity drain, so no condensate pump concern.

Look at the inside coils. I have seen A frames that look like a dirty filter because of air leaks around the air filter
Get a good view and clean if needed.