The best you can do is repot. Root prune back to healthy roots and use a well drained mix. Make sure it is not saturated until it recovers. fungicide is probably not needed but if you have any that treats phytopthera or root rots an application probably won't hurt and may even help.
Ginkgo is very resilient. It can tolerate very hard root pruning so don't be frightened to chop as hard as needed at this time of year.
Can also tolerate quite dry conditions despite the name 'maidenhair tree' which conjures up thoughts of ferns and damp.