backfire...can n-e one help??

i have a 90 toyota pickup sr5 that was rebuilt 2 weeks ago but i took the air intake out so i could put a new coupler on it and while i did that one of my spark plugs came completely off. i started my car back up after puting my intake back on not knowing that the spark plug came off and it started backfiring. i put the spark plug that fell of back on and it is still backfiring does anyone know what i can do to fix it?

it could be a problem with the timing belt, and if its fuel injected, then the fuel injection might be bad.

Hope this helps

Terrell