It looks like you are upgrading to a newer version of Gramps. The message is informing you that to proceed with the upgrade, Gramps will permanently change the old database to work with the newer version. So Gramps is asking you 'Did you make a backup of your database using the older version BEFORE proceeding with the upgrade." Once the database has been upgraded, the older version will no longer be able to read the files.
We all hope everything will work okay but to be safe, you should make the backup. I suggest a non-media backup. The changes gramps will make to the database will not affect the raw media files.
If you have the backup, you should feel safe to Proceed with the upgrade.
The alternative is to create a new empty database in the new version of gramps and then import the backup copy of the older version into to new empty database.
First of all, if you have 15 years of work in a single file with no backups, you're right to be angry--at yourself. You should make frequent backups and store them on multiple devices, including at least one in a different location from your primary one; so called cloud storage like Google Drive or Dropbox works pretty well for this.
Second, it doesn't say it won't open the file, it says it has to upgrade it first. If you don't have a backup already then you should reinstall the version of Gramps that you were using when you last worked on the file and make a backup. Then you can reinstall the newer version, open the file, and let it do the upgrades secure in the knowledge that if it fails you can go back to the previous version of Gramps and use your backup.