Basically check everything said before.
Also make sure you have enough fuel and that your FSD hasn't been turned off in the modules tab.
Here's a brief explanation:
Assuming that you're just setting off from a space port.
One of the usual reasons for not being to reach an objective is that the FSD does not have sufficient range. This happens because people swap one FSD for another type with a lower range than they had previously.
A second common reason is that people add heavy things to their ship which reduces the range that the FSD can take you.
Things that add mass include new modules, shielding, cargo and fuel.
It's best therefore not to plot a route before you make alterations to a ship or buy cargo and fuel.
I suggest you deselect your current route and try to see if the ship can plot a different path to your objective.
If it can't even get to the first star then you need to consider buying a better FSD and/or dumping modules and cargo.
If you're stuck half way through a journey.
You've plotted a route that includes a jump that the FSD can't make or you have insufficient fuel left for the jump.
Cancel your current jump and see if you can reach a port in your current system. Go to the port and refuel and replot the route.
If that doesn't work go back to the points mentioned above under starting from a spaceport.
If you can't reach a port in your current system try to reach another inhabited system. Inhabited systems usually have ports.