I would say that you cannot execute an RNAV missed approach, when the reason for going missed was a failure of the RNAV/GNSS system itself.
My understanding is that in this instance, you need to either revert to a conventional terrestrial aid (ie. VOR/NDB) or possibly Dead Reckoning navigation.
Originally Posted by https://skybrary.aero/bookshelf/books/2315.pdf
3.5.1.3 RNAV protected missed approach (not relying on conventional radionavigation means)
If the missed approach flight path is an RNAV procedure and if the approach has to be aborted for a reason independent of the operation of the RNAV/GNSS system, the crew must continue to use RNAV/GNSS system guidance to follow the missed approach procedure.
Extraction procedure
If the missed approach path is a RNAV procedure: The operator may have defined previously in their procedures, an emergency procedure for the case where the loss of RNAV guidance during the approach does not allow to follow the missed approach procedure provided for the considered runway. This emergency procedure will consist in following a dead reckoning path, allowing to reach a sufficient altitude and taking into account: - obstacles - aircraft performance - airspace restrictions
Note that you must continue to use the RNAV if the reason for aborting was
independent of the RNAV/GNSS system. However, if the reason was loss of RNAV guidance, you need an "extraction procedure", which is dead reckoning to a safe altitude.
I can't find a "proper" reference for any of this, but I haven't had a thorough search through the rules and regs as yet.