How can I fixScandinova freezer UF82C?

Details from index Plate Scandinova Type UF82C
Freezer A1348 0 115kg
NFS 240.
S 240. 540-6601150

My Scandinova frost free, front opening freezer gave the following symptoms a few days ago:
The interior started warming after the door had inadvertently been left slightly open for about half an hour and the warning light was flashing.
I closed the door and turned on the fast freeze, but after a few hours it was not getting any colder.
Freezer was emptied, turned off and left overnight. The following morning turned the freezer on again with fast freeze turned on; warning light still flashing and did not stop flashing until I turned off the fast freeze.
Internal temperature would not go lower than about -10c, but light still flashing.
The only way to stop flashing was turn off power for a few seconds, then turn on. That cycle repeated for a few hours, but seems to have settled now. Temperature will now not go below -15 at the top, but is -35c at the bottom. Any suggestions please and likely cost of repair.