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.

1. Start a Command Prompt
2. Type in netsh wlan show profile to list all WiFi network profiles that have been connected to, it may look similar to:

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

3. Identify which one you want the details for. Then type netsh wlan show profile "WiFi network name" key=clear
4. A long list of details should now appear. In the Security settings section, look for the Key Content field, which should be the password you are looking for

λ 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