New - Don't Beat Me Up Too Badly

See that little grey - sign? It indicates that this measure has less beats than the time signature asks for.
Either use Insert mode to insert a quarter ot use right-click into (an empty spot of) that measure > Measure properties > Actual duration

You will see a small "-" over the measure. That indicates it's duration is shorter than the time signature indicates. This is often what you want at the start of a piece for a pick up measure. You can make it four beats long by right clicking on the measure and selecting "measure properties". Then make the actual measure duration the same as the nominal duration.

If you arrived at this state while editing you may have inadvertently pressed CTRL+Delete in that first measure. That deletes not only a note but also the time associated with it. A plain old delete just deletes the note leaving silence (a rest).

If you are new, i suggest you should take a quick tour through the handbook https://musescore.org/en/handbook to familiarise yourself with some of the basic editing methods and to see what else is in there for future reference. As you obviously have successfully installed MuseScore, I suggest you read most of the section headed "Basics" as a start and skim the rest.

As mentioned, the "-" symbol is there to inform you that indeed this measure has been artificially shortened.

Since it seems you didn't shorten it on purpose, for future reference, there are two likely things that might have happened. Either:

1) You inadvertently selected 3/4 as the length of the pickup measure when creating the score (perhaps because that setting was leftover in the wizard from the previous score you created).

or

2) At some point you had a rest you didnb't want, and you discovered Ctrl+delete would eliminate it. That's true, but it eliminate the entire beat represented by the rest. So that's almos never what you really want. Instead the way to get rid of silence is to fill it with sound - =simply entering the new notes directly on top of the rest, or cut/paste notes from elsewhere to move them earlier or later, etc.