If your roof has extensive damage and fully repairing the roof has been ruled out, your two options would be re-roofing it (installing a new roof over the old one) or getting a complete replacement (removing the old roof and installing a new one). Of the two options, re-roofing is the least expensive, but not always the most cost-effective. If your existing roof has one layer and the roof deck is in good shape, re-roofing may be an option. If your roof has two layers, it should be replaced in most cases, and probably required in most areas.
While professional repairs can extend the life of your roofing system, your roof will reach a point where it can no longer be fixed. Trying to replace shingles on an asphalt roof over 25 years old is just costing you money that could be put towards a full replacement.