Why won't my 2003 Toyota Avalon start?

Check for proper spark , fuel pressure , injector pulse same as any no start . Something may have gone bad by pure coincidence . Check all the fuses

Is the security system active? Check the owners manual for the disarm procedure for your model.

Replacing a timing belt is something that a novice, or even a pro, can get wrong. You might have misread directions and misaligned a nub or three or completely misaligned the cam. Go back and check completely; maybe look at a youtube video on this.

Well lucky for yo I this v6 will forgive you, two teeth on belt will not allow engine to run. As valves are not fully closed. Some caution should be exercised on vvti engines as they can be interferance leading to head extraction. Rotate crank up to number one(TDC) check where the cam wheel marks are in realation to marks on tin backing. The tentioner will be a real bitch kitty to compress remove place in vise slowly apply pressure till hole in tentioner piston aligns w exterior holes add a small Allen OPPOSITE of installation side when removing pin be sure all slack is on tentioner side or will allow crank to rotate before cams causing same issue you already have. Hope this helps let us know if you get it dialed in.