My car won’t start or do anything

So, let me begin. I was driving on the highway
and all suddenly my engine losing power and
the screen was telling me to pull over and turn
off. So I did such that. I pull over on the side
and turn off. Then I try to turn it back on
pushing the brake down and pushing in the
button to start the car but my brake was
pushing forward while I was trying to push it
back. Then I just stop and let it beep and turn
it off. Now it won’t turn on or anything I
couldn’t even lock my door with out jumping
my car which the battery is in the back. And it
won’t do anything use my battery is hook on
to someone else is but even with it hook up to
someone else’s battery it won’t start the
engine... just the lights and AC.  I don’t know
what to do I’m afraid to take it to a mechanic
because of how much it would be.  I need
advice on what it could be and how much it
would cost.. if you can help me on that I
would be very thankful