Undo in blender

Ok so I am quite new to blender and around when I started I must of accidently pressed 'E' without realising whin the whole model was selected. Now i have a thin second layer on the front of my car, is there any way to get rid of this without losing all my progress??

 

Press CTRL+Z to undo.

 

Without losing all my progress?

 

CTRL+Z undoes on a step by step basis in order.

Say you've selected a vertex
you've deleted it
selected another vertex
pressed e to extrude
moved it in some x direction

If you press CTRL+Z it will undo the movement in the x direction, press it again, and it undoes the extrude, so on and so forth.
There is a limit to the number of steps that you can undo though. As long as extruding (What you did when you pressed e) your model was the latest thing you've done with it so far, CTRL+Z should reverse that. If you've moved the extruded pieces, then you actually would need to hit CTRL+Z twice. One to cancel out the movement, and the other to cancel out the extrude. Also, CTRL+SHIFT+Z redoes whatever you just undid with CTRL+Z.