I can't see my activities

I logged in to my account from another computer and I cannot see any of my previous activities. Yesterday, on the first computer, there was no last activity, the recording ended on the penultimate walk.
What’s happening?

The reason is that you have accidentally created two Strava accounts.

Fixing this can be a bit complicated - I have written up the process here: How do I fix accidentally creating two accounts at the same tracking service?