Hi
The Start and End tabs are found under the Clip Preview window and affect only the clip selected. You can re-select the clip in question (only a single clip can be adjusted at one time..not the whole timeline) and then drag the Start tab (red) to the left and the End tab (blue) to the right which will reset the clip in question to its former length. You can also do this with current sequence in the Sequence bin which should comprise the whole timeline.
The logical thing, and often stated on the forum, is to regularly Save project File As regularly giving a suitable different name each time. This will create a vpj file which can be used to reopen your project as it was at the save point.
If you pressed the tabs not too long ago then you may be able to backtrack to before that point with the Redo arrows at the top left of the top toolbar.
Nat