If the font used in your old Gimp .xcf file no longer exists on your computer then the the font in the text layer is replaced by a system default font. Gimp does not keep anything other than the font name in the .xcf file.
Go to https://sourceforge.net/projects/dejavu/...p/download
A 5.5 MB zip archive Unzip and put the contents of the ttf folder in your Gimp fonts folder. C:\Users\"yourname"\AppData\Roaming\GIMP\3.0\fonts
Whatever you do keep your old file safe and work with a backup. Open the backup and see if the text layer(s) are OK
Go to https://sourceforge.net/projects/dejavu/...p/download
A 5.5 MB zip archive Unzip and put the contents of the ttf folder in your Gimp fonts folder. C:\Users\"yourname"\AppData\Roaming\GIMP\3.0\fonts
Whatever you do keep your old file safe and work with a backup. Open the backup and see if the text layer(s) are OK