If you recently upgraded to WordPress 3.7.x, you might run into this error. It happened to my client’s site too, but I was able to fix it in about 15 minutes. This is what I did to fix it.
1) Logged into Cpanel
2) Went to site’s file manager > Laptops-tech.com (my client’s (laptop) site)
3) Downloaded fresh install of wordpress from wordpress.org.
4) Extracted the zip file downloaded in step 3
5) Replaced the wp-login.php in root and user.php in wp-includes.
6) Voila! I was able to log in.
This fixed the blank screen and login issue he was having after updating to 3.7.1.
PS: If this does not works, try deactivate all your plugins (by renaming plugins folder using file explorer).