A completely blank page means PHP stopped executing and error display is turned off. The error still exists — it is just hidden. Your job is to make it visible.
Step 1: read the error log
Open the Logs section of your dashboard and look at the PHP error log for the last few minutes. The fatal error names the exact file and line, which almost always identifies the offending plugin or theme.
Step 2: disable plugins without admin access
If you cannot log in, rename the wp-content/plugins folder to plugins-off over SFTP. WordPress will deactivate everything and the site should load. Rename it back, then re-enable plugins one at a time until the blank page returns.
Step 3: switch to a default theme
If disabling plugins did not help, rename your active theme folder. WordPress falls back to a default theme, which tells you the problem is theme code.
Step 4: check PHP memory
Memory exhaustion produces the same symptom. The log line will say Allowed memory size exhausted. Raise the limit from your dashboard, then find out what is consuming it rather than leaving the limit high forever.