I recently had a tune up on my car & then had gonefor a long drive. My engine light came on so I stopped turned off the car & opened the gas tank lid. After doing so my light did not come back on until just this morning. What would be the reason for it doing this??

It would be nice to know which code set the CEL. Otherwise it's just speculation, as the VW TDI does not have a sealed fuel tank where a vacuum leak would set the engine light.

Drive to nearest parts store and ask if they can read your code. I know Autozone does this for free. Windsolar

Once you have the DTC code from the OBD-II system, search the code at tdiclub.com to see what the indication is and how to fix it.

My autozones do not have the plug for a Tdi. The above mentioned site has a list
people that have the necessary plug and software to check the CEL, listed by state.