Inkscape will not start after an install or upgrade

Almost always leftovers from a previous version. Uninstall Inkscape from Settings → Apps, reboot, then install again. If it still fails, rename your preferences folder (%APPDATA%\inkscape) to inkscape.old and start the program: a fresh profile is created, and if that works, a corrupted preferences.xml was the culprit. Copy custom templates and palettes back selectively.

The splash appears, then nothing

Often a font cache problem. Start the program once from a command prompt to see the error text, and remove any recently installed broken font. Third-party font managers that keep fonts out of the Windows store can also confuse the text tool.

Extensions do nothing or report a Python error

Extensions run through the Python interpreter bundled with Inkscape. A separate Python installation on the PATH, or an antivirus tool blocking script execution in Program Files, breaks them. Check Extensions → the failing item and read the message in the status bar; reinstalling with the typical feature set restores the bundled interpreter. Extensions you added yourself belong in the user extensions folder inside %APPDATA%\inkscape.

The canvas lags or scrolling stutters

Look at the document, not the hardware: blur and other SVG filters, very large embedded bitmaps and paths with thousands of nodes all cost redraw time. Hide finished layers, link images instead of embedding while you draw, simplify paths with Ctrl+L, and switch the display mode to outline for heavy editing. Raising the rendering tile size in Preferences → Rendering helps on some machines.

A file will not open, or opens empty

Confirm what it actually is: many "SVG" downloads from craft marketplaces are ZIP archives or PDFs with the wrong extension. Open the file in a text editor — a real SVG starts with XML. Files saved by much newer Inkscape releases can use elements an older build ignores, which is one reason to keep a portable copy of the newer version around.

Text moves or reflows on another computer

The font is not installed there. Either supply the font, or select the text and use Path → Object to Path before sending the file, which converts letters to outlines. Do that on a copy, since the text is no longer editable afterwards.

Export looks wrong

Blurry PNG means the export DPI was too low; missing objects usually means an export area or selection restricted the output; unexpected color shifts point at the RGB-to-CMYK conversion at the print shop. The export guide covers each case.

Still stuck

Note your exact version from Help → About, the file that triggers it and the steps to reproduce, then search the project's issue tracker and community forum before filing a new report — the detail is what gets a bug fixed.