heat pump not cooling or heating, but compressor is running

Probably, That suction pressure equates to a -12°F coil temperature which I imagine is much more than 20° away from the environment it is in.

Before any freon is added, make a quick check for good air-flow on both coils, make sure you don't have a blocked filter.

Might try unloosening some cabinet screws on the indoor coil to make sure that the internal cabinet insulation hasn't broken loose and is blocking airflow.

Where are you? What season is it? Are you in COOLING mode? If yes, what's the temperature outside? If in heating mode, again, what is the temperature outside?

What is the indoor and outdoor temperature? What is your superheat? Amp draw?

When looking at the indoor and outdoor coils, is there any ice on the coils?

Does your reversing valve operate properly?

What is the td accross the coils (without supplemental heat being on)?

Have you looked at the operating instructions?

(When all else fails, look at the manual)

How old a unit is this? Is there a suction service valve? If so, try checking the compressor for leaking valves before putting a great deal of time or money into the fixing of the unit.

If you are low on freon, where has it been leaking from? The most obvious places would be schraeder valves and the suction accumulator (usually rusts out and has small pinholes).

If the system is 10 years old, or more, and it has an accumulator leak or leaking valves, replace the system. There are very efficient systems that will pay for themselves quickly.

Is the supply fan running? If you want to solve this problem please provide more information.