Make sure to follow these steps:

• Verify that PostgreSQL is still installed (eg. it should show in Add/Remove programs). Identify where it is: it is most likely in the folder of your previous Odoo install. Eg: C:\Program Files (x86)\Odoo 10.0e\PostgreSQL

• Open the Odoo configuration file in your new installation path. Eg: C:\Program Files (x86)\Odoo 11.0e\server\odoo.conf

• Find the pg_path variable and change it to the proper path, then save and close this file. (To avoid problems, do this with a text editor that is not going to change the text encoding or carriage return characters, Eg: Notepad++)
  pg_path = C:\Program Files (x86)\Odoo 10.0e\PostgreSQL\bin

• Go to the Windows Start menu and type 'services.msc'. Find the service of your old version (eg. Odoo 10) and stop it. Find the service of 'PostgreSQL_for_Odoo' and restart it. Find the service of your new version (eg. Odoo 11) and restart it.

• Open your browser in Incognito window and go to "localhost:8069". Hit CTRL+Shift+R to make sure you don't use anything in cache and to avoid that Odoo tries to automatically get into a DB created on the previous version (which would fail).

• Go to Manage databases and Create a new database to start. Give it a name, eg: odoo11test

• Open the configuration file (odoo.conf) again and replace the line 'db_name = False' with 'db_name = odoo11test' to force Odoo to use that database at startup.

• Restart again the services of your new version and of 'PostgreSQL_for_Odoo'.

• Open your browser with a regular window (not incognito) and go to "localhost:8069". Hit CTRL+Shift+R. It should work now.

• Open the configuration file (odoo.conf) again and change back the line to 'db_name = False'.
  

• If you still encounter an error, inspect (or ask an expert to inspect) the server log located at C:\Program Files (x86)\Odoo 11.0e\server\odoo.log