I apologize if these are stupid questions, but here goes: Is it possible you pinched or damaged one of the wires coming from the battery connections on the motherboard when you reassembled the case? Or, it's just a bad battery.

Your Dell laptop is showing a diagnostic blink code: 3 amber flashes followed by 5 white flashes, repeating in a loop. This code means the system failed to complete its startup self-check (POST). Depending on your model, the most common causes are: Memory (RAM) not detected or seated improperly - most common cause CMOS battery / Real-Time Clock (RTC) power loss Corrupted BIOS firmware Power rail or motherboard fault - less common The good news: most users can fix this at home in under 30 minutes. Follow the steps in order and stop as soon as your laptop boots. Before You Open the Laptop Work on a hard, non-static surface Touch metal first to discharge static Have a small Phillips screwdriver ready Refer to your model's service manual at dell.com/support 1. Hard Reset  Clears residual power — fixes many cases instantly. Unplug adapter, remove battery (if removable) Hold power button for 30 seconds Reconnect power and try booting 2. Reseat the RAM - Most Common Fix  Unscrew the memory access panel on the bottom Push the side clips outward — RAM stick pops up Firmly press it back in until clips snap Two sticks? Test each one alone to find a bad module 3. Reset CMOS  Unplug everything (adapter, USBs, peripherals) Hold power button for 60 seconds Plug in and retry 4. BIOS Recovery  For corrupted firmware. Unplug the adapter Hold CTRL + Esc, then plug adapter back in Keep holding until the recovery screen appears → select Recover BIOS 5. Run Built-In Diagnostics  Power on → tap F12 repeatedly → select Diagnostics Next Steps If the issue persists after these steps, it may indicate a motherboard failure or a deeper power subsystem fault. We recommend contacting our support team for further assistance. You can use the Get Help Now chat feature located at the bottom right corner of this page. Or, you may reach out to a trusted technician for a hands-on diagnosis. Please see this video for general No Post Troubleshooting.

That code means a bad battery.  You'll need a replacement. https://www.parts-people.com/index.php?action=item&id=19380

Hi @divyendusingh · It's an LCD panel issue. · Most likely there is internal damage in the panel that you can't see. · It happens a lot with panels that are cracked as shown in your image. · You can't fix them, they have to be replaced. · Use the manual to view the procedure to remove the display panel and then once it is out, check on the back of the panel for the part number of the panel. · Search online for the part number, to find suppliers that suit you best. That way you'll be sure that you're getting the correct panel for the model.

It is a battery issue. This forum post has more information that may help.

Memory or Read-Only Memory (ROM) failure If competent, take the memory out and try the following - if there are two DIMMS then try each one in each slot to see if there is any change, if just one DIMM the try both slots and also try no RAM installed and it should produce a 2,3 code. Manual at https://dl.dell.com/content/manual36483742-dell-g15-5515-service-manual.pdf?language=en-us and memory at P. 24 Remember to insert and remove at 45 degree angle. Good luck Got warranty?