Most likely the cartridge has failed, can be DIY if knowledgeable? IF not just call plumber as suggested, Mike Scott pretty reliable.
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single-handle shower that won't turn off is often due to a faulty cartridge or a problem with the valve itself. To fix it, you'll need to first shut off the water supply, then remove the handle and access the cartridge or valve. Replacing the faulty part should resolve the issue.
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