Project files download issue

Recently some files in the system suddenly became unavailable for download. When I click in the attachment, there is error message on the page "We are sorry, but ProjectPier is not currently able to execute your request. An Error Report has been sent to the administrator."

Also, I get the following error in the application log:-
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#1 FATAL: exception 'FileNotInRepositoryError' with message 'File 'c8f925c8ea2d5f1c321a43cdeb2d94daaf63af6f' can not be found in the repository' in /var/www/html/projectpier/library/filerepository/backend/FileRepository_Backend_FileSystem.class.php:68
Stack trace:
#0 /var/www/html/projectpier/library/filerepository/FileRepository.class.php(58): FileRepository_Backend_FileSystem->getFileContent('c8f925c8ea2d5f1...')
#1 /var/www/html/projectpier/application/models/project_file_revisions/ProjectFileRevision.class.php(110): FileRepository::getFileContent('c8f925c8ea2d5f1...')
#2 /var/www/html/projectpier/application/models/project_files/ProjectFile.class.php(233): ProjectFileRevision->getFileContent()
#3 /var/www/html/projectpier/application/controllers/FilesController.class.php(328): ProjectFile->getFileContent()
#4 /var/www/html/projectpier/environment/classes/controller/Controller.class.php(69): FilesController->download_file()
#5 /var/www/html/projectpier/environment/classes/controller/PageController.class.php(62): Controller->execute('download_file')
#6 /var/www/html/projectpier/environment/classes/Env.class.php(119): PageController->execute('download_file')
#7 /var/www/html/projectpier/init.php(138): Env::executeAction('files', 'download_file')
#8 /var/www/html/projectpier/index.php(9): require('/var/www/html/p...')
#9 {main}
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These file attachments were working fine before and everybody was able to download them.
Please help!!