this is a known problem with Canon IXUS 400's and Canon will* fix it for free no matter how old it is:

https://self-service-kb.canon-europe.com/SRVS/CGI-BIN/WEBCGI.EXE/,/?St=37,E=0000000000405489546,K=1153,Sxi=15,Case=obj(45929)

*possibly
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Mk2a RioCar 120Gb - now sold to the owner of my old car
Rio Karma - now on ebay...