ODO meter display zero?

[Icosphere]

Hi there,

Due to the Pandemic lockdown for over 3 months, I haven't driven my car since and so when I tried starting it up, the battery was completely dead. I jump started it first 2 goes and successfully drove it for at least 30 minutes, hoping the battery would charge. Although haven't really driven it anywhere at length, the other day I tried to drive it again but it wouldn't ignite so I jump started it again but this time I notice the Odometer was showing zero. I seem to have lost my mileage information.

Does anyone have experience of this? What should I do to fix it? How much would be expected to get this fixed?

Thanks