There several reason that can cause this. One is dirt filter which restricts air flow cause it to freeze up. If you have clogged filter change and give time to thaw out and turn back on. If this works or doesn't call for a tech to look at. If you been running with clogged filter pretty good chance your coil is dirt which will slow cooling and will be less efficient. Remember always turn unit off to thaw before tech comes, he can not work on frozen
Outdoor Fan Not Running
If notice outdoor fan not running but compressor is turn off. This will make your compressor over heat and fail possibly. If caught right away could be no more than capacitor. Do not try to change. Call for tech. Capacitor can still be loaded with energy even with power off.