Firstly,
Update your version of putty to the latest version and try again. Older versions of Putty will not be able to connect to the current version of OSMC as Debian deprecated some older insecure SSH keying schemes in a recent update.
If you are sure it is a password problem (I don’t see how) then do the following:
Shut down and remove your SD card, insert it into an SD reader in a PC.
Edit cmdline.txt and add the word recovery to the end of the line, save, and boot off the SD card.
This will take you directly to a root recovery console - you will need a keyboard plugged into your Pi. Type:
passwd osmc
And retype your password. Then edit cmdline.txt to remove the word recovery:
nano /boot/cmdline.txt
Once you have done that type exit to reboot.
If this does not work, your issue is not that the password is wrong.
Edit: fixed typo. passwd root should be passwd osmc of course.