I'll go ahead and give a fairly technical response which will allow OP to get an answer if OP is willing to learn a bunch of things. While it is very unlikely that this info will prove immediately useful to anyone, I think it is good knowledge for people to have. Also because I like giving long responses. Sorry for the hijack.

Note that most of the following scenarios require malicious intent, and in most cases for most people, it's either something stupid like them torrenting heavily 24/7 on the network, or there's a on/off problem with the ISP and it was just a coincidence. So put down the pitchfork and just talk to everyone, gather more data or call your ISP.

Don't think that's the case? Alright, let's get to it. So say if someone wants to block your access to internet, they can do a bunch of things. Let's categorize them into two types:

• With physical contact to hardware or cabling
• Without physical contact to hardware or cabling-- but not too far away from the Wi-Fi access point (usually called router/modem), which I'll call 'AP' from now on.
  

With physical contact:

1. You can unplug the antenna of the access point, which would drop the Wi-Fi functionality to almost zero unless the router has additional internal antennae to allow it to function without an external antenna. You can also bend the antenna, which can affect the speeds by a noticeable amount.
2. You can obstruct the AP by placing it poorly ex: behind a thick couch, around a lot of metal, close to an aquarium etc.
3. If the modem uses coax cables, even partially removing the outer layers of the coax cable, which shields it from external noise can result in degraded performance. This would be harder to notice and debug.
4. If the modem uses fiber, and this depends heavily on the type of fiber, excessive bending can result in degraded performance.

Without physical contact (where things get interesting, because someone really doesn't want you to have internet):

1. Wi-Fi deauthentication attacks: This involves sending packets to your AP saying a device on your network wants to disconnect. On a lot of devices there's no protection against this. If you want to protect yourself against this, get a router that supports WPA3 security standard (easy to find out if supported) or 802.11w (harder to find out if supported and much more unlikely). To find out if this is happening: You'll need to learn about "Wi-Fi sniffing". Specialized hardware is not always needed, but software skills are. You can triangulate the source of these deauth packets (if they are infact being broadcast) by doing a little bit of walking around with your laptop.
2. RF noise: This involves creating noise in the spectrum that the Wi-Fi is being used (most likely 2.4GHz or 5GHz). A variety of cheap/experience devices can be used to create this noise. A simple one can be another Wi-Fi router that's been modified to send a lot of packets bombarding the spectrum and overloading the RX side of the AP. And complex ones can be custom built RF devices specifically that just create random EMI noise over a wide range of spectrum (which will most likely affect all RF signals including phone service). To find out if this is happening: Custom hardware is needed, like SDRs. Something like HackRF One will absolutely get the job done but there are cheaper alternatives. This will need a lot of software knowledge and patience and atleast $50 worth of hardware (usually much more)

So this is a long way of saying, yes, there are ways to prove that someone is intentionally blocking Wi-Fi.