Wondering if anyone on here can help as I'm completely stumped.

So I removed the timing belt on my 1.6 TDCI Duratorq engine (TZJA) so that I could replace the high pressure fuel pump. Everything was completely normal at this point. After replacing the pump (which I'm pretty sure has nothing to do with it?) I went to put the timing belt back on. I rotated the crankshaft to align the locking pin and then went to align the camshaft with it's locking pin but it wouldn't turn. It's a kind of springy stop not a hard stop. I thought maybe the valves where interfering with the pistons so I rotated the crankshaft back so that all the pistons were half way down the bores and tried to rotate the camshaft again but still the same problem.

Hopefully that all makes sense. I can't work out what is going on, can anybody help?

Assume the crankshaft can turn freely so pistons not touching valves etc or locking anything?
Then what else could cause camshaft to lock?  - if you've changed the fuel pump, does it need to be timed correctly, take it out and see if that is the issue as otherwise there's only opening valves that are driven by camshaft (unless it's DOHC in which case, there's the vacuum pump?)
Assume there are no other belts or chains connected?
As I don't know this engine, hopefully others can comment?

7 hours ago, MikeJeffreys said:

I rotated the crankshaft to align the locking pin and then went to align the camshaft with it's locking pin but it wouldn't turn.

Do we understand from that, that you didn't put the crankshaft and camshaft timing pins in before you removed the belt ?