ЕImmediately after upgrading Joomla from 3.6.x to 3.7.x the main error pops up:
404 Component not found
How to solve this problem:
1) make a rollback to a backup before the update (if there is no possibility – manually transfer the database and files to a fresh installation of Joomla and manually install all the components).
2) _before update_ (this is important!) go to the main menu of admin area: Extensions -> Extension Manager -> Search
3) select everything there (or just all “Fields” if you don’t know what’s what; most likely the problem is because of them) and click “Install” (top left).
4) go to “Extensions” -> “Extension Manager” -> “Databases”
5) Click “Fix”.