Why does your three point hitch not work when you unhook your front end loader?

Both are hydraulic. Often when you disconnect one hydraulic device you bleed air into the system... You might need to cap off the front end loader hydraulic connectors, bleed the air out and refill with hydraulic fluid. Another possibility is that you are simply low on fluid. Yet another, less loved cause, is a stuck priority diverter valve - that prioritizes hydraulic pressure when both devices are attached.

Finally, not knowing your particular model - the possibility does exist that there is no diverter and that it runs off the same hydraulic line as the loader - in that case, the fix would be just to connect a line to the front end loader inlet, then connect the other line to the outlet (tractor side connections, clearly not the loader itself).

Hope this helps.