Before starting, make sure you are signed into the same Google account with the same username that you used to upload the pictures. Note that your email address associated with your Google account can be a non-Google email address.
If you don't remember the password of your Google account, try the account recovery tool and answer the questions as best as you can. Estimated dates are better than nothing. You can try the tool several times. Try the Can't sign in to your Google Account troubleshooter.
If you cannot find photos uploaded to Google Photos, check these locations:
Google Photos library
Google Photos Albums
Read about Albums that do not appear at Google Photos here
"Recently added" area of Google Photos
Trash at Google Photos (if it's been over 60 days since deletion, it won't be possible to recover a deleted file)
Album Archive (this site also contains your retired Picasa Web Albums)
Photos added to Blogger using "insert image" can be found in the "Photos from Blogger" album, but not at Google Photos.
Photos deleted from "Photos from Blogger" will no longer appear at the Blogger website
See this Album Archive FAQ for why some albums at Album Archive may not appear at Google Photos
Trash at Google Drive. If you already have cleared the Drive Trash, contact the Drive Team: https://support.google.com/drive/?hl=en&authuser=0#topic=14940&contact=1
Scroll down to "Contact Us" which goes here: https://support.google.com/drive/gethelp" Tell the Drive team that you permanently deleted files in Google Drive (do not mention "Google Photos"). Effective July 2019, the option to contact the Drive team may no longer help to restore deleted files at Google Photos: https://www.blog.google/products/photos/simplifying-google-photos-and-google-drive/
Read the "Find missing photos & videos" support page
Read "Search in Google photos"
Try other browsers to access your albums or library. These are some common browsers:
Try browsing in Incognito mode (Chrome) / Private Browsing (Firefox)
Disable extensions/add-ons/plug-ins
On Android phones, search at Google Play for deleted photo recovery apps; for iPhones you could try these methods
Look for your files at any other cloud storage websites and apps, such as Dropbox, iCloud, OneDrive, Verizon backup, etc.
If you ever shared the missing photos with anyone, ask them to share the photos back with you.
If any of the photos were on your computer, check the computer's recycle bin.
If you backed up to external hard drives or USB sticks, check there.
Check your phone to see if Backup & Sync successfully synced:
Open the Google Photos app
In the Assistant, look in the bottom left corner to see if it says “Backup complete”
Wait and see if the problem resolves itself in a few hours or days in case the problem was caused by a temporary Google server problem.
If someone else had access to your Google account (or your phone was stolen): Make your account more secure