Your Windows computer will remember the WiFi passwords for every network it's been connected to (on the assumption it used a password and not something like a certificate or other authentication mechanism).
Now while you might think they've disappeared into the ether never to be seen again, it is possible.
C:\> netsh wlan show profile
Profiles on interface Wi-Fi:
Group policy profiles (read only)
---------------------------------
<None>
User profiles
-------------
All User Profile : Network1
All User Profile : Network2
Security settings
section, look for the Key Content
field, which should be the password you are looking forλ netsh wlan show profile "Network1" key=clear
Profile Network1 on interface Wi-Fi:
=======================================================================
Applied: All User Profile
Profile information
-------------------
Version : 1
Type : Wireless LAN
Name : Network1
Control options :
Connection mode : Connect automatically
Network broadcast : Connect only if this network is broadcasting
AutoSwitch : Do not switch to other networks
MAC Randomization : Disabled
Connectivity settings
---------------------
Number of SSIDs : 1
SSID name : "Network1"
Network type : Infrastructure
Radio type : [ Any Radio Type ]
Vendor extension : Not present
Security settings
-----------------
Authentication : WPA2-Personal
Cipher : CCMP
Authentication : WPA2-Personal
Cipher : GCMP
Security key : Present
Key Content : <password here>
Cost settings
-------------
Cost : Unrestricted
Congested : No
Approaching Data Limit : No
Over Data Limit : No
Roaming : No
Cost Source : Default
Ref: CNET