Threats of website being "shut down" by users (thr
Scan your computer for viruses, watch your logs, etc.
An attack on a linode might actually get you some help since it will impact other users as well. I can't speak for Linode, though. I hope they'd help a little if a(n innocent) customer needed some serious expert help. Locking down SSH is good…maybe you should lock it down a little harder than normal for awhile (only allow yourself, or create a new user just for this time period with a totally new password). But most likely they'll just DOS or DDOS you. Are we talking script kiddies or anonymous? A good firewall might help, there's an iptables module "recent" that CSF can use to limit floods from a single IP. Not useful for DDOS though. At that point you probably need help from the guys that manage the network hardware.
Keep an ssh session open, maybe.
If you're hosting multiple sites, and you're OK with the target site going down for awhile, point the DNS somewhere else, maybe another linode with a static web page, until you can get a handle on things.
If you don't have fail2ban or CSF/LFD running, install now and limit login attempts to 3 and permblock anything above that. You can always relax it later. Wordpress->login lockdown. HTTP basic auth -> LFD can catch those. Stop miniserv.pl if it's running…go command line for awhile.
Someone smarter will note if this is crap advice. I hope it's all an idle threat. For many of us this would be the equivalent to throwing a 6-gallon molotov cocktail of napalm through a retail store window.
sudo ufw limit ssh
Also, fully update whatever web app you're using, such as Drupal or WordPress. Disable unnecessary plugins and update the rest. SQL injection is by far the easiest way to attack a web site these days, so you absolutely don't want to be running a vulnerable web app.
On the other hand, if you're using an outdated app that can't be updated – such as Gallery 2 or phpBB 2 -- then you might as well turn off the site now and work on a full upgrade.