If it ran fine before you took it apart, then you did something wrong when reassembling it. Either you broke something, or you are putting it together wrong.
Did you mishandle the armature, muscle it out of the motor, poke at it with sharp instruments? If not, it's probably OK. You can still damage it if you want to by doing the things I mentioned, but armatures don't just go bad from sitting.
Don't get too bogged down investigating odd/unusual things that could be wrong. Look carefully at every step you took.
Did you change the wiring? How are the brush springs? Are they relatively springy? Are the brushes (little carbon cylinders) in place? Is the brush plate mounted correctly? Is the reverse switch or E-unit clean?
Just look at the stuff you touched or disassembled. Don't go looking for other things. Just look at the actions you took.
