I hope you also made a backup of the original database for the 2.8.6 before attempting upgrade.
But ... the error you are seeing now means the upgrade did take place - and the directories you copied into new code directory is the issue now. So hope you didn't over write those directories ... theme, blocks, mod - in the new code directory.
IF you over wrote the new code directories, download the 2.8.11+ zip again locally and unzip. Then upload (replacing) the theme,blocks, mod directories from that local unzipped 2.8.11+. Then check ownership/permissions on all new code directories/files. Am guessing those should match the owerships/permissions settings of the moodleold code directory.
If you had any addons - ie, theme, mod, block - that was not part of core, *copy* those themes ,mods, or blocks, to the new code directories. Note that's *copy*. So if you had a 'mytheme' directory in the oldcode/theme/ directory, copy **just** 'mytheme' directory to the newcodedirectory/theme/. Same for anything in mod or blocks like that to their respective mod/block in the new code directory.
That should get your code back to where it should be.
In the config.php file of the site, add the debug lines you can find in config-dist.php to the config.php file. There is a section in config-dist.php for debugging ... you need only those debug lines. There is no need to restart services, one just needs to hit the site again with browser and let's see what Moodle might report. If it shows anything, copy and paste the errors which are displayed in text to a local text editor. Then edit the local text file to obscure anything that might be sensitive information that you shouldn't share back here - things like paths /home/[customerlogin]/blah/blah - replacing real customer name with something like [customerlogin]. Then come back here and post info.
It would help to know where hosted (you did say 'shared host') and other details like operating system and versions of PHP and MySQL (or the DB using).
'spirit of sharing', Ken
