If you have a recent dump of the master database, follow the instructions in the SA Guide for restoring a master database from dump: Recovering the master database
If you don't have a recent dump of master, do you have a login that can bcp out the major system tables from the master database? If so, you can follow the method documented in the old 12.5 Troubleshooting Guide: Manually Rebuilding Adaptive Server with bcp
If you can't even get the contents of the system tables, you can create a new server, use the DISK REINIT command to rebuild sysdevices using the device names from the server errorlog without overwriting those devices. Then use the DISK REFIT command which will rebuild the sysdatabases and sysusages tables by scanning the contents of the devices. syslogins can then be rebuilt from account names in the sysusers tables of your user databases, but the paswords will be lost, you will have to assign new passwords. There is still various small stuff to do after this (remote servers, remote logins, password security configurations, etc.), but the solid core will now be back up.