If a message just sits in Outbox for more than a few minutes, the most common culprits are a weak connection or a browser extension interfering with the request. Try opening Gmail in an incognito window — if the email goes through, disable extensions one by one to find the bad actor. Also check that your attachment isn't over 25 MB, which is the hard limit for sending.
Before assuming something is broken, check your Spam and All Mail folders — filters sometimes misroute messages aggressively. If expected emails are missing from there too, ask the sender to verify they used the right address. On the account side, go to Settings → Filters and Blocked Addresses and make sure nothing is accidentally redirecting or deleting incoming mail.
This usually comes down to browser cache or a temporary issue with Google's content delivery. Force-refresh the page with Ctrl+Shift+R, and if images still don't appear, check that 'Show images' is enabled under Settings → General → External images. For attachments that fail to download, try right-clicking and using 'Save link as' instead of the default download button.
A wrong password is obvious, but the less obvious cause is an outdated recovery method blocking automated verification. If you're hitting 'Couldn't sign you in', check that your device's date and time are set to automatic — clock skew breaks OAuth flows. For two-factor authentication problems, use backup codes found under your Google Account security settings.