Fixing 'The User Profile Service Failed the Logon' on Windows

“The User Profile Service failed the logon. User profile cannot be loaded” blocks you from logging in entirely — but the underlying cause is usually a corrupted registry entry for your specific user profile, fixable without losing your actual files.

Step 1: log in with a different administrator account, if one exists

You’ll need access to a working account (a separate administrator account, or Safe Mode with an account that still works) to fix the affected profile’s registry entries.

Step 2: boot into Safe Mode if no other account is available

Restart while holding Shift, then:
Troubleshoot → Advanced options → Startup Settings → Restart →
  press 4 or F4 for Safe Mode

Step 3: open the registry editor and locate the ProfileList key

regedit
Navigate to:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList

Step 4: identify your affected profile among the SID subkeys

Each subkey here is named after a Security Identifier (SID) corresponding to one user account. Look for a SID subkey, and check whether there’s also a matching subkey with .bak appended to it (e.g., both S-1-5-21-...-1001 and S-1-5-21-...-1001.bak) — this specific pattern, two related keys where one is a .bak variant, is the most common signature of this exact problem.

Click each SID subkey and check its State value:

A State value of 0 generally indicates a healthy profile.
A non-zero value (commonly 128 or similar) indicates the
profile Windows considers corrupted or incomplete.

Step 6: fix the key pair, if you found the `.bak` pattern from Step 4

1. Rename the (non-.bak) broken-looking key, e.g. append
   ".ba" temporarily
2. Rename the .bak key by removing ".bak" from its name
3. Rename the key from step 1 to append ".bak" instead
4. Set the RefCount and State values on the now-active
   (renamed) key to 0

This effectively swaps which of the two related registry entries Windows treats as the authoritative one for that profile — resolving the specific inconsistency that produces this error, without touching the actual user files on disk at all.

Step 7: restart and attempt to log in normally

shutdown /r /t 0

Step 8: if the registry fix doesn’t resolve it, create a new profile and migrate data manually

1. Create a new local user account
2. Log in as the new user once, to let Windows generate a
   fresh profile
3. Copy files from the old profile's folder (still present
   under C:\Users\) into the new profile's corresponding
   folders

This is a more manual fallback — your actual files under the old profile folder are still there and copyable even when the profile itself won’t load, since the files aren’t what’s corrupted; the registry pointer to the profile is.

Why this error is a registry-tracking problem, not a file-loss problem

The core of this error is Windows’ bookkeeping about which profile belongs to which user getting into an inconsistent state — not the actual user data being damaged or lost. Understanding that distinction is what makes the registry-level fix in Steps 4–6 the right first approach, rather than assuming the affected user’s files themselves need to be recovered from a backup.