If you are within the time limits for bringing a case, you can make an employment tribunal claim for disability discrimination under the Equality Act 2010.
To do so, you will need specialist legal advice, available through your union, local law centre, Citizens Advice Bureau or other source. Long-term absence from work is very likely to stem from a disability. Many of the disability claims taken to employment tribunals have involved employees dismissed from their job after long-term absence.
Time limits for claims in the employment tribunal are very short, so make sure you act quickly.