I have hot water base board heat. About a week ago, I noticed I couldn't get the house to go above 62 degrees. When I touch the baseboards the two rooms at the beginning of the loop are hot, but as I go farther through the house the units are colder and colder leaving the two bedrooms freezing cold. Last winter I had a problem with a pipe freezing, so now I keep a small heater on in the basement to avoid this happening again. The circulating pump comes on and is only a year old. As I said the problem started suddenly. What could be wrong? Can I fix it myself?

Air in the system? Bleed baseboard system.
Low boiler water pressure? Raise water pressure

These two items are the cause of the condition you are experiencing.

There can be others but not likely.

As well, check that the zone valve is open ( if you have one) and the pump is running ? To be noted, just because a pump is running does not mean it is moving water, because of system pressure or air as hvac1000 stated.......you may have a frozen pipe section again...

I had the same problem and it was air in the line. There should be bleeders on each heater at one end. If the pump is on all the time just crack that little valve open and see if air comes out. When water starts to come shut off.