the distance you can jump depends on the mass of your ship. If you change your outfitting or put wares in your cargo hold, you can't jump as far as you could when you where +/- empty. However, that should not make a difference as huge as you're describing.
Are you sure you're heading back to your old system? because there is a known bug:
Some station names are used multiple times and when you're doing missions, the mission marker not only pops up on the station you actually have to go to (station X at System Y) but also on any other system with a station of the same name. (Station X at System Y, Station X at system F, Station X at system N) so when you have a look around on your Galaxy-map you have several Mission markers on differen systems all of a sudden. - usually not too close by though)
idk if the bug also affects the actual mission system, so you could return the mission in the "wrong" starsystem or not. But it's definately worth a check.
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updating again .. i found this " tonnes " option and i got my 4 jumps back ... what is this tonne ?
with that you can simulate your cargo as already filled. afaik it uses your current cargo filling as default when opening the galaxy map.
p.s. it is also possible that one of your old jumps went through an empty field of space with no system for intermediate jumps in beween. So you kinda have to take the long way around, now that your ship is full.