ReadyNas 526x - Cannot access and re map drive

Before anyone can help you, you need to answer StephenB's other question:

"And your PC is assigned a 192.168.1.x address?"

You can find out the IP of your PC using the methods listed here: https://support.microsoft.com/en-us/windows/find-your-ip-address-in-windows-f21a9bbc-c582-55cd-35e0-73431160a1b9.

Chances are, it's not, being either a 192.168.0.x or 10.x.x.x, and that is your problem.  All devices must be in the same sub-net to connect to each other.  Assigning a static IP is a bad idea precisely because of what just happened to you.  It is much better to reserve an IP address for the NAS in your router.

If I am correct, then you have a few options:

1. Temporarily assign a static IP to your PC that's also in the 192.168.1.x range (see https://support.microsoft.com/en-us/windows/change-tcp-ip-settings-bd0a07af-15f5-cd6a-363f-ca2b6f391ace).  Then when you can get into the GUI, change from static IP to DHCP and re-start the NAS.  Then don't forget to revert back to DHCP on your PC.
2.   Perform an OS re-install on the NAS (NOT a factory default).  That will also change the IP selection to DHCP but will not harm your data (but accidently doing a factory default instead will).
3.   Go into the router interface and change it's subnet to 192.168.1.x (often shown as 192.168.1.0/24 or 192.168.1.0 with a network mask of 255.255.255.0).  You may or may not have to re-start the router and devices connected to it.

After you have done this, then go into the router interface and reserve an IP address for your NAS and again re-start the NAS.  If you want to assign an address that's already taken, you'll have to turn the other device off or disconnect it from the network before you can.  Turning it off is better so there is no possibility of conflict when you re-connect it.

If your PC already has an IP in the 192.168.1.x range, then you have a different issue.  I doubt that's the case, but if it is, come back with more details and we'll try again.