There are multiple ways to contest deletion depending on the process being used. You may be looking for:

• Wikipedia:Requests for undeletion, for contesting uncontroversial deletions done through the simple proposed deletion or some speedy deletion processes
• Wikipedia:Deletion review, for contesting controversial or previously discussed decisions made at the main deletion venues
• Wikipedia:Proposed deletion § Objecting, for contesting an open proposed deletion
• Wikipedia:Articles for deletion § How to contribute, for contesting an open nomination at Articles for deletion

To contest another process, you may be looking for:

• Wikipedia:Appealing a block
• Wikipedia:Closing discussions § Challenging a closure
• Wikipedia:Verifiability § Responsibility for providing citations

• Wikipedia:Contests