There was a malfunction at Trunkses time machine...

[withheldforprivacy]

...and took him to a wrong era? (select an era of your liking and tell the story)

 

[Captain Cadaver]

He'd simply return to his own time immediately and wait 3 years to refuel and try again, unless the malfunction traps him in that era. The additional 3 years needed on top of that time may mean that the cyborgs may kill Bulma during that time, however.

If we assume he has to stay in the era for whatever reason, he goes back to the beginning of the series, meets Bulma before she meets Goku and we get Back to the Future - Dragon Ball edition.