Some 14 million computers may have upgraded to Windows 10 barely 24 hours after the launch, but it seems Microsoft may have a major problem at hand. Users who upgraded to the latest OS are complaining of issues with the much-touted Start Menu and Cortana.

Customers who made the switch to Windows 10 have taken to Microsoft support Community to share the slew of issues they have been battling since the upgrade.

Most of the users are tackling problems with Cortana and the Start menu not launching. Every time a launch is attempted, users see a "Critical Error" message saying that the functions are not working and a fix will be attempted the next time the user signs in.

Workarounds, such as rebooting or refreshing the system, making a new account, or using other accounts, have all failed for a majority of people who upgraded to Windows 10.

"So Installed Windows early this morning and once I was able to log in onto my personal account. I got a pop up saying '*Critical Error* Start Menu and Cortana aren't working. We'll try to fix it the next time you sign in.' I read online to try a different account. I have tried that with numerous accounts and I still have the problem," complained a user who upgraded from Windows 8.1 to Windows 10 on the Microsoft Forum.

Another user also shared: "Upgraded my system yesterday and it was working well until an hour ago. I decided to restart my system and after that, the problem occurred to me as well. Please help. I need it to be fixed as soon as possible."

Some users even attempted to go back to their older OS to resolve the issue but to no avail.

"It seems like my PC is also not responding to any keyboard shortcuts. Was planning to revert back to 8.1 but only then I found out that the setting is not working as well! Help please! I don't want to get stuck like this!" ranted another user.

Microsoft has acknowledged the issue and Jen, who works for Microsoft, stated that they are looking into the matter and that a fix is on the way.

"We've fixed many of the underlying causes for this issue, but there are still some eluding us, and I'm sorry to hear that this is impacting you guys. We have telemetry and crash reports that we're watching for this," noted Jen on the Community forum.

It is not known when the fix will be pushed out. For now, users can only file a report and alert Microsoft of the bug.

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