Assuming you had it working a couple of months ago, as you said, and no settings have been changed in the mean time, then it shouldn't be too hard.
- reboot the pidp-11, just to make sure. Sometimes the WiFi becomes unstable after a while, specially with older Pi models.
- the easiest way is to temporarily hook up a screen and keyboard to the Pi, if booting into the GUI first open a terminal window on the Pi, and type 'ifconfig'. it will show you the IP addresses ('inet') of the configured network interfaces, including wifi ('wlan0' probably). Then you can open a remote shell to the pidp-11. You want to specify the user 'pi' with the connection.
- if the above is not an option, then the harder way is to use a network scanner and try to identify the pi from the found nodes on your subnet. A free simple one is more than enough for this purpose.
- which terminal program you use, from whatever common platform (Windows, Linux, Mac), should not matter. Once you have found the Pi's IP address you can ping it to check if it can be reached in the first place (assuming you don't have a super restrictive firewall running on the PI...).