I had to changet the timing belt in my truck and now it won't start

think you have not put the timing back right and it will need to be re done. hope you used a new belt and adjuster when you put it back together. you need to re line the timing marks on cams and on crank. then put the belt on adjust up making sure the number 1 piston is at the top as well as the timing marks on the cam check this with the crank making, put belt on and adjust up then turn engine one full turn. no spark plugs fitted. back to the timing marks making sure they line up if not re adjust the time until it line back up every time you turn the engine one full turn. then put the spark plugs in etc and start enging.

Did you put the belt right ,how long was down ,battery need charge .

are you certain you got all the timing marks lined up properly?
If not then you've bent the valves again. These things have close tollerances. Get a compression tester and check all cylinders have good compression. If so then line up all your timing marks and check to see if the rotor button is pointing at the right spot to fire cylinder 1. Did you seperate the plug leads from the dizzy? If so check they're in the right order.

recheck your timing marks and try again something is not lined up right..

Chances are your cam sprocket is off one tooth, bring the engine to Top dead center #1 cylinder firing stroke lining up the flywheel timing pointer notch to the timing marked zero. remove the timing belt cover to check that the timing marks line up if not remove tensioner and reset the sprocket so it lines up then reinstall tensioner recheck.

Was the belt you replaced broke? Did it run before you replaced the belt?

The cam pulleys turn 3 or more turns to one of the camshaft. You need to find someone that is experienced on this vehicle. Just aligning marks does not mean its in sync.

In the back of the engine you will find a rubber plug that covers a hole in the bell housing. Remove the plug and on the flywheel there are timing marks. you have to turn the engine over clock wise untill you see them. There are 3 lines, line up the middle line with the pointer, put the left cam shaft sprocket mark straight down and the right cam sprocket mark straight up, and install the belts.

Try replacing the fuel filter.In a more old school find your fuel line and make sure you have flow.

Why do you think the timing belt is an issue, you would be better to give me the symptom rather the question you posed, you can check the timing belt by pulling of the belt cover and putting the crankshaft pulley to the TDC mark and then checking the index mark on the cam pulley(s), the pulley mark should line up with the index mark on the engine, if not the belt has jumped, in this case it must be replaced, if you do replace the belt make sure all the pulleys and the belt tensioners bearings are not rough feeling or have play in them.