I am on a main boiler system that is connected to multiple units. I replaced a hose cap on my release valve to stop a leak well as I bleed the pipes for bubbles. Prior to my tasks today i had no heat. When I bleed my pipes for bubbles the pipes started to warm up but when I close the bleeders my pipes get cold again. Any ideas why this would be?

prob. still have air in pipes

Air as already been said. Weak or bad pump is also a possibility.

This has a strange, familiar ring to it. Do you have a temperature control dial mounted on the baseboard unit?

No, I do not have a temp. reader on my baseboard. :( I'm affraid I'm not much help..

If the system has isolation valves for purging, someone may have inadvertently closed one or forgot to re-open after purging. Did anyone else in the building complain about heating recently?

I asked around but as fare a I know, no one else has stated a heating issue. A great thought though, I will all out maintenance line in the morning and see if they have done any work in the past few days. I van also see if they can check the boiler for any problems.