My Mountain bike was working perfectly last week. I went for a ride at the weekend and the rear derailleur has developed a problem. There is no spring in the derailleur. The lowest jockey wheel has moved towards the crank and the chain is sagging, previously the lowest jockey was properly inline with the upper jockey wheel. The cable is still in the same position as it was when the derailleur was working correctly. Feedback appreciated as I need to go for a ride.

Pictures would help, as would knowing what type of derailleur it is. Sounds like the spring could have broken in the derailleur body, you could try making sure that it is good and clean. Try pushing it by hand and seeing if there is any spring tension fighting you.

Was any type of maintenance done, the derailleur or wheel removed, or did you have any type of incident while riding just before the problem arose? It can feel like the spring is broken if the derailleur was allowed to rotate or was reinstalled so that it is not sprung against the stop on the hanger.

I agree with Higgins that it sounds like a broken derailleur spring.

I have tried pushing the derailleur and there is no spring tension. I was thinking a spring must have broken inside the body but wasn't sure. Its a SRAM X7 Derailleur. Photo attached.

Time for a new RD.

It is possible the spring broke, but if you ask for help it's considered a part of the process to answer diagnostic questions (Post #3 above). Depending on the answer you may save yourself the cost of a new derailleur.

No maintenance completed on the bike for a few weeks, as I said previously it was working fine before the ride were the rear derailleur became faulty/stopped working correctly. No incident happened while riding. I was only 2 minutes into the ride.

OK, thank you for clarification. It would seem then that the spring is the logical culprit.

Yes looking in the window of the RD you should see a short what some people call a "screen door" type spring. Your LBS might have one they kept from a bent or broken RD.