To fix a collapsed entrance that is an ore tile, simply paint terrain as grass, then back to rock and it'll clear the ore vein so you can tunnel into it.

To create a flat mine entrance:

Method 1:

• Align the floor and ceiling corner heights (inside) with those of the entrance tile (outside).
• Mine out from the inside.

Method 2:

• Tunnel in, which puts you down 20 slope, tunnel up the next tile inside (reversing the 20 slope), then mine one tile in to get a flat tile. Fill in the down and up slope tiles, then mine straight out.