My heart just started beating really fast?

I had the same fast heartbeat (over 120 per min) for no particular reason. I'd had an enjoyable time with my friends and returned home. Managed to persuade my husband to take me to A&E. There I was given an injection and hospitalised overnight. The cardiologist put me on Tenormin to be taken daily. I kept this up for some years but have slowly weaned myself off them as I've had no symptoms since. I think a lot depends on how long the attack lasts. I was in my early sixties when this happened.