Recover from Postgres FATAL: could not open file pg_tblspc/
The very first thing to do is to make a copy of whatever data files you still have, and to keep it and any backups safe until long after your recovery effort is complete.  Please read this (short) Wiki page:

http://wiki.postgresql.org/wiki/Corruption

Once you have done that, you can attempt various recovery strategies without fear that you will be worse off for the attempt, beyond the time required to try it.  In general I recommend carefully following one of the techniques described in the documentation -- attempts to cut corners or to be creative often lead to corruption.  Only a seasoned expert with a good understanding of PostgreSQL internals should attempt to deviate from the documented steps.

You didn't describe your backup strategy; details of what is available there may suggest alternatives you would not otherwise have.

Ultimately, if you have data of value which is not backed up, you may need to hand-edit the system tables to eliminate references to lost tablespace.  This is not for the faint of heart.  There are a number of companies with which you can contract for such services, many of whom have experience with recovery from catastrophic hardware failure like this.

http://www.postgresql.org/support/professional_support/

I am not affiliated with any of these companies.