I observed the same issue.
I refreshed the console page ( CTRL + F5 ) and I was able to see the 'Activate Cloud Shell' button again !!
However, if above does not resolves the issue, some tips:
From google docs Google Cloud Shell Limitations
Weekly usage: Cloud Shell also has weekly usage limits. If you reach
your usage limit, you'll need to wait until the specified time (listed
under Usage Quota, found under the three dots menu icon) before you
can use Cloud Shell again.
Also, there are usage limits on this, check your email if you had violated any conditions on shell usage:
Warning: Violating the Terms of Service will result in Cloud Shell
being disabled for your account. This constitutes activity that
adversely impacts Google Cloud Platform services, other customers' or
their end users' use of services, or the Google network used to
provide these services. Coin mining and network scanning using Cloud
Shell are strictly prohibited.