Reproduction steps (on an unaltered default theme)
Go to your Inbox
Click 'Disable my inbox' at the base of the message listing
a prompt appears indicating the following, click OK...inbox now disabled
On a desktop...
the Messages icon now indicates that the inbox is disabled (greyed out/hover message displayed)
Click the button and the inbox appears.
On a mobile...
tap the 3-line/burger menu
the Messages icon now indicates that the inbox is disabled (greyed out)
tap the icon, and the inbox appears
However, that's not the only way to get to the inbox...
tap Account -> Messages
a prompt appears indicating the following:
Click OK and your inbox is now re-enabled.