If your unit is blowing 'warm air', there are a couple of things to do to

discover what the problem is and whether or not you should turn the

unit off till someone gets there to take care of the problem.

If the unit started blowing warm air during a normal cycle and you

just happened to notice it, turn the unit off for 45 minutes, then

adjust the temperature on the t-stat and CLICK the Heat/Cool

switch to Cool.  This prevents any bouncing of the mercury in

the stat, (if of manual variety typically), and overheating the

components.

If the unit starts cooling, keep an eye on it for the next few hours,

particularly if its in the middle of the heat of the day.  If it does

it again, turn the unit off untill someone gets there to take care

of the problem.  DO NOT KEEP OVERHEATING THE UNIT.

You'll make a small problem a BIG one and you will regret it.

If it runs out ok for several hours, it was probably either someone

adjusting the thermostat bouncing the mercury, making the unit

try to restart too quickly or a power 'flicker'.

Keep a watchful eye on it for the next couple of days.  If the problem

repeats itself, call someone.  If not, Great!

If you just adjusted the thermostat prior to noticing the unit is not

cooling, turn the unit off for 45 minutes and restart it using the

same method above, (setting the temp first, then clicking to

Cool).  The mercury probably 'bounced' and tried to restart

the compressor too quickly.  This should take care of the

problem and in the future, adjust your temperature settings

very carefully.  This is an incredibly common problem.

If neither of these above situations match yours, you may

have a power problem.  If the unit is on the ground, go outside

after turning it on and see if the outdoor section, (condensor),

is running.  If not, check the fuses and/or breaker. 

If you find a fuse blown or the breaker tripped, replace the

fuse and/or reset the breaker and restart the unit using the

above mentioned method.  Watch the unit carefully for the

next few hours.  If the problem repeats itself or if the unit

doesn't start, contact a repair person.  Continuing to run

the unit will only creat much larger and expensive problems.

As I have time I'll modify this information to be more complete.