Hi Gerardo and welcome to the forum.
Are other sound sources e.g. speakers, headphone working OK and only the internal microphone is not working?
Can you look at alsamixer.
Open a terminal session by typing Ctrl+Alt+T , then type:
alsamixer
In alsamixer, hit F6 to list/select soundcards.
Select a analogue soundcard.
If that is not seen easily from the F6 list, post a screenshot.
In alsamixer, hit F5 to display ALL sound channels.
Look for any Mic channels showing as [MM] , which means muted.
Use < > arrow keys to move to the muted channel and hit M to unmute. Then up arrow to increase mic volume.