Why won't my heat work in my 2008

Did you check the actual radiator prior to changing the thermostat? If NO, there was most likely air in the system, thus the lack of heat.

Changing the thermostat, introduced even more air into the engine cooling loop, as it seems you did not purge trapped air from the system.

What coolant did you use? You really should use Toyota SLLC (Super Long Life Coolant).

If you have more than 100K miles, may be a good idea to change out the water pump, since you already changed the thermostat.

READ this tread in its entirety! All questions will be answered, as there are useful links within this thread.
How to replace engine coolant pump and thermostat | PriusChat

The airlift type coolant tools ($86-$128) make coolant drain&fill easy; no more trapped air due to the vacuum the tool creates. An air compressor is required, if you don't already have one.
Amazon.com: air lift coolant

If you don't want to buy the airlift, the Lisle coolant funnel ($25-$30) is a must.
Amazon.com: lissle coolant funnel

 

In addition to the above, which is definitely the place to start, there may be a fault with an electric pump in the heater loop, that boosts the coolant to the heater core when the engine isn't running. The thread referred to above mentions the Mini VCI diagnostic tool ($25 and an old XP laptop). It's a good tool that gives you testing control on the electric water pumps, reads coolant temp, and makes bleeding much easier. (You can properly bleed the Prius with that and a short length of tubing attached to the upper radiator bleeder valve.)