The HP BATTERY ALERT appears and the PC doesn't start up when I press the ENTER button.

[Sara99]

When I turn on my PC the HP BATTERY ALERT appears and when I press ENTER button it doesn't start up and return to that screen (as shown in the 2nd pic). It's not the first time that HP BATTERY ALERT appears but usually when it appears and press ENTER the pc will startup I don't know why this happened this time. I want to know should I replace my battery and what type should I buy or could I solve this problem without replacing my battery 

[CherylG]

@Sara99 

I have occasionally seen a bad battery prevent startup. You could remove it and see if it will startup without it. No harm in running without a battery-I assume you use it on power anyway since the battery is bad?

The Maintenance and Service Guide gives part numbers for batteries. Your model and the 4720S use the same battery. Chapter 4 page 55 shows the part numbers for all compatible batteries which you can use to Google search for a replacement.

https://support.hp.com/us-en/product/hp-probook-4520s-notebook-pc/4145198/manuals

Examples of replacement batteries:

https://www.walmart.com/ip/Superb-Choice-6-cell-HP-593576-001-Laptop-Battery/36863000

https://www.laptopbatteryexpress.com/HP-593576-001-battery-p/hp-815xt-593576.htm

[Sara99]

@CherylG 

Thank you for your respond.

I did change the battery because none of the methods you mentioned worked with me, fortunately the "hp battery alert" didn't show again but the problem is that the laptop won't start up 

First the screen will be like this

Then it will turn to black like this, after that it will go back as the first image and so on

I'm a student and my laptop is so important because it has all my projects and such things so I hope that there is a quick solution that won't let me lose my files.

[CherylG]

Immediately after powering on start tapping the Esc key. This will open a menu where F2 System Diagnostics is an option. Tap F2 key and run the tests. If you don't have F2 option use F10 to enter BIOS and look for Diagnostics in there. Looks like you may have a hard drive problem.

[Sara99]

@CherylG I tried this several times. The diagnostic starts well but then the laptop will suddenly shut down some people told me that I should reset it to factory settings but there are some important files that I don't want to lose so is there any other options? Or can I at least get back those important files? 

[CherylG]

I don't think you can reset it to factory settings if you  upgraded to Windows 10. You have Win 10 listed as current OS in your first post and that is not what it shipped with. Of course you could still try it but usually upgrading to Win 10 corrupts the Recovery partition. Unless of course you have Recovery Media?

Actually, I am afraid the laptop would shut down trying to reset to factory also. Probably the best thing to do before tring anything else is recovering your files. Remove the hard drive, hook it to a USB HDD Adapter and plug it into usb port of a working computer. If the drive is ok you should be able to browse it for files and copy them over to drive of host machine.

An alternative to removing the drive is booting with a Linux cd/usb and copying the files to external drive:

https://www.howtogeek.com/howto/31804/the-10-cleverest-ways-to-use-linux-to-fix-your-windows-pc/

Chapter 4, pg 87 of the manual shows how to remove the drive:

http://h10032.www1.hp.com/ctg/Manual/c02439213

Example of adapter:

https://www.amazon.com/StarTech-com-SATA-Drive-Adapter-Cable/dp/B00HJZJI84