Hi my 2 cents worth,
1) Maintain last cleared altitude and track. !!! Take note of MSA in the TMA, which would have been briefed.
2) Troubleshoot !! Try all other form of communications, other VHFs, HF, CPDLC, mobile phone.
3)SQK 7600.
If by this time all else fail, I am pretty sure ATC already found out about your problem, be it Mode-C or Primary radar.
4)I would track back to a holding fix of the STAR make 1 hold and then shoot the approach. This is to buy some time for myself as well as for the ATC to co-ordinate.
Any comments?
Cheers,
lion-g